1. Field of the Invention
The invention relates to a method for the production of synthetic quartz glass, to an apparatus for the practice of the method, and to the use of the synthetic quartz glass.
2. Discussion of the Prior Art
For the manufacture of light-conductive fibers having a core and a jacket, it is desirable, especially when high-purity quartz glass is used, to have available a jacket material which has a lower refractive index than the core material. To this end it was proposed in French Patent Application No. 2,208,127 to use as the jacket material a quartz glass doped either with boron oxide or with fluorine. The fluorine-doped quartz glass is obtained by oxidizing silicon tetrafluoride in accordance with the equation SiF.sub.4 +2H.sub.2 O+O.sub.2 .fwdarw.SiO.sub.2 +4HF, whereby small amounts of fluorine are to be incorporated into SiO.sub.2. However, the oxidation can also be brought about by reactive methods in which no hydrogen or H.sub.2 O is present, such as for example in the high-frequency plasma, so that no hydrofluoric acid will form.
It is clear that it is not possible by the disclosed method to produce a fluorine-doped quartz glass in which the fluorine doping will result in a predetermined reduction of the refractive index below that of high-purity quartz glass. It is in this light that one must also interpret the statement made in U.S. Pat. No. 3,869,194 that the differences achieved in the refractive indexes of core material and fluorine-doped jacket material in a light-conductive fiber are so slight that such fibers are not suitable for optical signal transmission.
The production of synthetic quartz glass which is substantially free of "water" and thus free of absorption bands at wavelengths of 1.4, 2.2 and 2.7 .mu.m--hereinafter referred to as "hydroxyl-ion-free"--is known from German Pat. No. 1,208,740. The hydroxyl-ion-free quartz glass is obtained by oxidizing a hydrogen-free silicon compound in a hydrogen-free gas stream containing elemental and/or bound oxygen, and depositing the oxidation product as a vitreous mass on a refractory support, the gas stream being passed through an induction-coupled plasma burner.
Accordingly, it is an object of this invention to provide a repeatable process for the production of a synthetic, hydroxyl-ion-free quartz glass especially such a quartz glass having a refractive index of n.sub.D .ltoreq.1.4570. It is especially an object of this invention to provide such a process for the production of synthetic hydroxyl-free quartz glass having a fluorine content of at least 4 g per kg of synthesized SiO.sub.2.